The problem with known methods of controlling the flow of material when deep-drawing a workpiece is that an accumulation of material can form in the flange regions of the workpiece during the deep-drawing process. If so, this leads to the flow of material out of the flange region of the workpiece being reduced or even stopped, so that the material from the regions already deep-drawn, for example the border or the base, thins out during continued deep-drawing, and consequently the risk of a crack forming in these regions presents itself. This is particularly the case if the draw depth is very great or if the workpiece is to be deep-drawn into a drawn part with more strongly bent regions, such as corners for example.
On the other hand, the accuracy of the shape of the base can be impaired by constriction of the base region of the drawn part from the border which occurs during the deep-drawing process. This is because when the base structures are formed in this case there are hardly any regions of pre-stretched material. This can then have a particularly adverse effect if the base surface is distinctly smaller than the border surface.
In order to prevent cracks, workpieces can be used, the diameters of which do not exceed the maximum diameter which is predetermined by the respective maximum drawing ratio related to a swift cup test. However, the deep-drawing method then can only be used for producing drawn parts in specific size ranges. Moreover, the flange regions of the workpiece are comparatively narrow, so that increased care has to be taken during the deep-drawing process. In order to prevent breaks the deep-drawing process can also be carried out in several steps during which the cross-section of the deep-drawing punch is gradually reduced. In this way, the condition predetermined by the maximum drawing ratio can be adhered to longer even when using workpieces with large diameters. However, the tool complexity is increased by this procedure. Furthermore, the cycle time during manufacture is lengthened, whereby the efficiency of the method is reduced. Although facilitating the continued flow of material from the flange region into the deep-drawn regions by reducing the friction of the workpiece, which fundamentally inhibits the flow process, against the deep-drawing die and/or blank holder by using lubricants can assist in carrying out the process correctly, it cannot fully guarantee this, yet. Moreover, additional cleaning procedures are required for the drawn part and the deep-drawing tools when using lubricants. In the flange region of the workpiece, the direction of flow of the material during the deep-drawing process can be slowed down over a particular section by correspondingly arranged braking beads. As a result, the workpiece material can already be ironed before the actual deep-drawing process. However, only the evenness of ironing the material is achieved by this procedure. The enlargement of the entry radius of the workpiece from the guiding section into the forming section of the deep-drawing die is also not given. This can lead to unwanted wrinkle formation on the drawn part.
According to patent specification EP 1 526 931 B1, the material flow is controlled when deep-drawing sheet metal by stamping a Z-shaped blocking step into the edge of the sheet metal, which inhibits the material flow. The blocking step is reduced again in the course of the deep-drawing process, so that the flow of material from the flange region into the die is facilitated. However, an additional deflection of the material and correspondingly elaborate tools, which have to exert high additional forces, are required for this procedure. Moreover, friction is increased by the deflection.